For the sauce, whisk all the ingredients in a bowl until well combined. Taste and adjust any of the ingredients as needed. Set aside.
Fill a saute pan about 1/4 of the way with water over medium heat, cover with a lid and bring to a simmer. Add the dumplings to the water and turn down the heat to low. Cover with a lid and let the dumplings steep in the water until warmed through. (Refer to the instructions on the package for cooking time and instruction).
Preheat the oven broiler on low. Drizzle the crostini with oil and season with salt and pepper. Broil until golden brown and crispy.
Season the tuna on both sides with salt and pepper. Coat on both sides with sesame seeds, gently pressing so they stick to the tuna and coat well.
Heat a nonstick pan or skillet on medium high heat. Once the pan is hot, sear the tuna on both sides golden brown for about 45 seconds per side or to your desired temperature using a thermometer (see below). Remove from the pan and set aside to rest for 5 minutes before slicing against the grain.
Serve the tuna with dumplings. Spoon the sauce over top. Garnish with scallion. Serve with crostini.